This invention relates to a prepreg for printed circuit boards excellent in thinness and denseness of printed circuit boards mounting electronic parts, said prepreg being able to be attached or bound to a carrier film or a copper foil, and processes for producing the prepreg.
Recently, with a tendency of miniaturization and light weight of electronic appliances, a demand for printed circuit boards with a thinner type, a high wiring density and high electronic parts mounting density, surface smoothness for forming very fine wiring and low production cost has been raised year after year.
As insulating materials and interlaminar adhesive materials for printed circuit boards, there have been used prepregs obtained by impregnating woven base materials such as glass cloth, Kevlar cloth, etc. or non-woven base materials such as glass paper, aramide paper, etc. with a thermosetting resin such as an epoxy resin, a polyimide resin, a phenol resin, etc.
Using such prepregs, various methods for obtaining thinner printed circuit boards have been studied. For example, generally used prepregs have a thickness of about 100 to 200 .mu.m mainly. In order to obtain prepregs having a thickness of 30 .mu.m after pressed with heating, the distance of stands constituting glass strand used for glass cloth is made large, the resin content is increased and finer stands constituted by finer glass strands are used. But such prepregs have various problems. For example, since the volume percentage of glass strands is low, rigidity decreases and dimensional stability during the step of formation of circuits is lowered. Further, since the glass strands are easily unevenly distributed, concaves and convexes are brought about depending on the presence or no presence of the glass strands. In addition, since the unevenness on a interlayer circuit board is not able to be flattened due to thinness of the board, the unevenness easily appears on the surface after lamination. Moreover, since the rigidity of thin glass cloth per se is low, the glass cloth is easily broken by impregnation with a resin, resulting in making the cost for material higher.
When a prepreg obtained by using such a glass cloth is used as an outer layer in a multilayer printed circuit board, the unevenness of interlayer circuit appears on the surface of the outer layer, unless the thickness of prepreg is made twice or more as large as that of the interlayer copper foil, resulting in making it difficult to obtain the desired thinness.
Further, multilayer printed circuit boards produced by using the prepregs using such glass cloth also have problems in that there easily take place lost of core center of a drill, and damage of the drill during drilling using the drill having a small diameter due to the glass cloth unevenly distributed.
Even if laser drilling is conducted, straight advance of laser beams is lowered due to the presence of glass fibers and drilling precision is lowered. Further, there is another problem of poor surface smoothness due to unevenness of interlayer circuit easily appearing on the surface. Therefore, so long as the prepreg containing such glass cloth base material is used, it is impossible to obtain thinner multilayer printed circuit boards with high wiring density.
Under such circumstances, it is proposed to use a prepreg of insulating resin without containing reinforcing fibers such as glass cloth in place of the prepreg containing glass cloth (e.g. JP-A 6-200216, JP-A 6-242465, etc.).
For example, U.S. Pat. No. 4,543,295 discloses the use of a thermoplastic polyimide adhesive film; JP-A 4-120135 discloses the use of a high molecular weight epoxy resin film having an average molecular weight of 70,000 or more; JP-A 6-200216 discloses the prepreg obtained by introducing silicone units into a polyimide resin rich in film-forming ability; JP-A 4-29393, JP-A 4-36366 and JP-A 4-41581 disclose the prepregs obtained by using as resins acrylonitrile-butadiene rubber/phenol resin, phenol resin/butyral resin, acrylonitrile-butadiene rubber/epoxy resin. Further JP-A 6-196862 discloses a copper foil clad adhesive film obtained by adhering an adhesive film of semi-cured film-forming resin without containing glass cloth to a copper foil.
These prior art techniques, however, have the following problems.
According to U.S. Pat. No. 4,543,295, since the thermoplastic polyimide adhesive film has an adhesion temperature of 250.degree. C., which temperature is higher than the heating temperature of about 170.degree. C. generally employed for printed circuit boards, the insulating material is limited to highly heat resistant polyimide, etc.
According to JP-A 4-120135 and JP-A 6-200216, solvents for diluting the resins are limited to those having high boiling points, resulting in making solvent removal efficiency low after coating and drying for obtaining a film. When the solvent is retained, properties of the resulting printed circuit boards are often damaged.
According to JP-A 4-29393, JP-A 4-36366 and JP-A 4-41581, the insulating materials used therein are poor in chemical resistance, and heat resistance, so that the resulting printed circuit boards are poor in heat resistance and electrical insulating properties.
Further, since these insulating materials do not contain reinforcing fibers, rigidity is low compared with known prepregs. Thus, dimensional stability during the steps for producing printed circuit boards is also low. Further, since the thermal expansion coefficient is extremely larger than those of conductor circuits and electronic parts to be mounted when a printed circuit board is made, breakage of connection portions by soldering easily takes place due to thermal expansion and shrinkage by heating and cooling.
Thus, the above-mentioned insulating materials containing no reinforcing fibers can only be used as interlaminar insulating layers in multilayer printed circuit boards, at present. But, it is necessary to take the low rigidity and large thermal expansion coefficient into consideration when such insulating materials are used. Thus, it is difficult to reduce the production cost.